This invention relates to a magnetic head mounted in a magnetic recording system, more specifically to a magnetoresistive head and a fabricating method thereof to read information recorded on a magnetic recording medium.
Recently, a GMR (Giant Magnetoresistive) head using a so-called “spin valve”, which has a basic configuration consisting of magnetic layer/nonmagnetic conductive layer/magnetic layer/antiferromagnetic layer as written in JP-A 358310/1992, has been widely adopted for a magnetoresistive head mounted in a magnetic recording system as a read sensor. In the spin valve, a magnetic film in which the magnetization direction is pinned in one direction by magnetic exchange coupling with an antiferromagnetic film is called a pinned layer. On the other hand, another magnetic layer is called a free layer because the magnetization direction can be changed in response to an external field.
A GMR head using a spin valve transduces a magnetic signal to a voltage change or a current change by using the phenomenon in which the electrical resistivity changes according to the angle of magnetization between the pinned layer and the free layer. Therefore, it is most important that the magnetization direction of the pinned layer be fixed unidirectionally (concretely, the direction perpendicular to the magnetic recording medium) when making the spin valve function as a magnetic sensor. That is, it is necessary that the magnetic field required to reverse the magnetization of the pinned layer (which corresponds to a magnetic exchange coupling field given by the antiferromagnetic layer) be controlled to be sufficiently larger than the signal field from the magnetic recording medium, the leakage field from the write head, and so on. Moreover, considering the annealing process during magnetic head fabrication and the operating environmental temperature of the head, the thermal stability of the magnetic exchange coupling field, which is imparted to the pinned layer from the antiferromagnetic layer, also becomes an important factor.
Currently, a large magnetic exchange coupling field can be obtained, and, because of their excellent thermal stability, alloys designated by Mn-M1 containing about 50 at % Mn (where M1 is a noble metal such as Pt etc.) have come into mainstream use for the antiferromagnetic film. These materials do not impart a magnetic exchange coupling field to the pinned layer on the as-deposited spin valve. This is because a Mn-M1 alloy is a disordered alloy having an FCC structure as-deposited and does not exhibit antiferromagnetism. In the case where a magnetic exchange coupling field is imparted to the pinned layer, it is generally necessary to apply annealing under magnetic field. It is known that a Mn-M1 alloy phase-transforms to an ordered alloy having a Cu—Au I type structure by annealing at a temperature around 230° C.˜270° C., and becomes antiferromagnetic. Additionally, while the annealing is carried out in a magnetic field, the pinned layer exchange-couples with the antiferromagnetic layer unidirectionally and can pin the magnetization direction. That is, in the case where a Mn-M1 alloy is used for the antiferromagnetic film to control the magnetization direction of the pinned layer, an annealing step in a magnetic field is necessary to obtain a large magnetic exchange coupling field and an excellent thermal stability.
In general, it is preferable that the upper and lower shields which are placed sandwiching the spin valve in the direction of the film thickness, the free layer being one component of the spin valve, and the magnetic pole at the write head have magnetization directions controlled to be the in direction of the track width which is perpendicular to the magnetization of the pinned layer (the direction perpendicular to the magnetic recording medium). Therefore, there is a concern that annealing in a magnetic field to control the magnetization direction of the pinned layer affects the above-mentioned other magnetic layers, specifically the magnetization direction of the free layer. If the magnetization direction of the free layer is shifted from the desired the track width direction, problems arise such as destroying the symmetrical nature of the response characteristics relative to an external field and not being able to obtain excellent reading characteristics. Therefore, it is necessary to pay close attention to control the magnetization direction between the pinned layer and the free layer to be orthogonal.
Moreover, because a Mn-M1 alloy (where M1 is a noble metal such as Pt, etc.) needs a film thickness of at least about 10˜20 nm, one expects there to be a disadvantage in terms of making the distance between the upper and lower shields narrower with increasing recording density.
As another means to pin the magnetization direction of the pinned layer, a configuration of a pinned layer is disclosed in the patent document JP-A 7235/1996, in which a pinned layer configuration not using an antiferromagnetic layer is described consisting of a magnetic layer/antiferromagnetic coupling layer/magnetic layer which is so-called “self-pinned”. Moreover, in the patent documents JP-A 57538/2000 and JP-A 251224/2000, a configuration is disclosed in which a hard magnetic material is used as one of the above-mentioned magnetic layers in order to pin the magnetization of the “self-pinned” pinned layer more strongly. In this configuration, the annealing process in field is not necessary to obtain magnetic exchange coupling with the antiferromagnetic layer. Additionally, because there is no antiferromagnetic layer, the total film thickness of the spin valve can be made thinner, with the resultant advantage that the gap between the upper and lower shields can be made narrower. However, there is no detailed description concerning the direction control of the magnetization of the pinned layer and free layer presented above, and there is no guarantee of normal operation after the thermal and magnetic histories during fabrication of a magnetic head.
Moreover, it is necessary to apply a longitudinal biasing field to a GMR head using a spin valve to give a multi-domain structure to the free layer to keep out Barkhausen noise created when a free layer becomes a multi-magnetic domain structure. A longitudinal biasing method is disclosed in the patent document JP-A 57223/1995, in which a hard magnetic material or a laminated layer of a magnetic layer and an antiferromagnetic layer is placed at both sides of the spin valve and a longitudinal biasing field is applied to the free layer to make it a single domain structure. In particular, the former is called a hard-bias structure, and it has become the mainstream of present GMR head structures.
The hard-bias structure is effective in keeping out Barkhausen noise. On the other hand, it is well known that the structure pins the magnetization of the free layer at the edges of the sensor and forms a so-called insensitive area. Because the magnetization direction of the free layer is hard to change in response to a signal field in the insensitive area, formation of the insensitive area substantially lowers reading sensitivity. Especially, this problem is expected to become more conspicuous with progressive narrowing of the track width and increasing occupation ratio of the insensitive area as the longitudinal recording density of magnetic recording systems increase in the future. Moreover, since the magnetization direction of the pinned layer is shifted due to a longitudinal biasing field caused by the hard magnetic material, there is concern that it will decrease reading output and affect the symmetry of the reading waveform.
If the longitudinal biasing field applied to the free layer is reduced by, for instance, thinning the film thickness of the hard magnetic material to solve these problems, the effectiveness of keeping out Barkhausen noise becomes inadequate. This means that keeping out Barkhausen noise and insuring reading sensitivity have a trade-off relationship, and it is difficult to satisfy both at the same time.
Another means to apply a longitudinal biasing field to the free layer is disclosed in the patent document JP-A 250205/2001, in which a laminated bias film consisting of laminated films of a bias nonmagnetic film/bias magnetic film/bias antiferromagnetic film is fabricated connected to the free layer after forming the spin valve consisting of an antiferromagnetic layer/pinned layer/nonmagnetic conductive film/free layer. In this configuration, the magnetization direction of the longitudinal biasing magnetic film is pinned by magnetic exchange coupling with the longitudinal biasing antiferromagnetic film. Moreover, a longitudinal biasing field can substantially be applied to the free layer by coupling the free layer with the longitudinal biasing magnetic layer magnetically or antiferromagnetically through the longitudinal biasing nonmagnetic film. In this case, one expects that there is an advantage that the longitudinal biasing field can be easily adjusted by controlling the film thickness of the longitudinal biasing nonmagnetic film. However, in this configuration, it is very difficult to produce an orthogonal array of the magnetization of the pinned layer and longitudinal biasing magnetic layer. That is, because two annealing steps in a magnetic field are necessary in which magnetization of the pinned layer is pinned in a direction perpendicular to the magnetic recording medium in the first step and magnetization of the longitudinal biasing magnetic film is pinned in the track width direction in the second step, a problem arises in which the magnetization direction of the pinned layer is shifted to the track width direction from a direction perpendicular to the magnetic recording medium after the second annealing step. In order to pin the magnetization of the pinned layer and longitudinal biasing magnetic film in orthogonal directions, it is necessary to trade-off the magnitude of the magnetic exchange coupling applied to either of the aforementioned magnetic films and their thermal stability. In this case, it may be very difficult to obtain reading characteristics with high reliability.